1. Introduction

Solid state lasers operating around 2 μm are of great interest for many applications, such as medicine, environment monitoring and lidar systems [1]. The wavelengths above 2 μm are suitable for pumping ZnGeP₂ (ZGP) optical parametric oscillators to efficiently generate 3-5 μm mid-infrared radiation via non-linear frequency conversion [2]. Holmium lasers reach longer wavelengths such as 2.09 μm in Ho:YAG [3–5]. The pump wavelength required for Ho³⁺ lasers is 1.9 μm, for which the high power pumping laser diodes (LD) are commercially unavailable. The Ho³⁺ lasers are directly pumped by a Tm:YLF laser (pumped by a 796 nm LD), where the lasing around 1.9 μm is the transition from the first excited state ³F₄ to the ground state ³H₆ of Tm³⁺.

Rare earth sesquioxides have a high thermal conductivity compared with YAG [6]. Fortunately, Tm³⁺ doped rare earth sesquioxides [7–9], Lu₂O₃ and Sc₂O₃, show very large Stark splittings in the ground state manifolds due to the strong crystal field strength in the sesquioxides, enabling the ³F₄-³H₆ transition to produce a long wavelength emission band around 2.1 μm. The ³F₄ state can be populated upon ³H₄ excitation around 800 nm through cross relaxation with a neighboring Tm³⁺ in the ground state, resulting in quantum conversion of one 800 nm photon into two 2.1 μm photons [10]. The unique optical properties of Tm³⁺ doped sesquioxides enable lasing around 2.1 μm to be directly generated by 800 nm LD pump with the theoretical limit for the slope conversion efficiency to 76%. Koopmann and associates [7–9] reported, for the first time, continuous-wave (CW) laser operation at 2.065 μm in Tm³⁺: Lu₂O₃ and 2.116 μm in Tm³⁺:Sc₂O₃ single crystals pumped by a 800 nm LD. An output power of 75 W and the slope efficiency of 40% were achieved in Tm:Lu₂O₃ [7], indicating Tm³⁺ doped sesquioxides could become an alternative to Ho:YAG lasers. These sesquioxide single crystals, however, are difficult to produce due to a high melting temperature (~ 2450 °С) [8]. Sintering of sesquioxide transparent ceramics has, therefore, attracted tremendous attention because the sintering temperature is much lower than the melting temperature [11,12]. Antipov and associates realized significant CW laser operation at 2.068 μm of Tm³⁺:Lu₂O₃ ceramics pumped by 800 nm LDs [13–15]. An output power of 34 W with a slope efficiency of 44% were achieved [15].

Compared with Tm³⁺:Lu₂O₃, Tm³⁺: (Lu,Sc)₂O₃ mixed crystals can reach a longer emission wavelength [16], which is beneficial for efficient generation of mid-infrared radiation via pumping ZGP. Meanwhile, the mixed crystals provide a disordered crystal structure and thus inhomogeneously broadened optical spectra of Tm³⁺ compared with Tm³⁺ in Lu₂O₃ single crystals [16]. The inhomogeneous broadening can compress the pulse width in mode-locked operation on one hand and improve the matching of the absorption band with the pump laser wavelength on the other hand. Very recently, CW laser operations around 2 μm in some Tm³⁺ doped mixed sesquioxide ceramics pumped by 800 nm LDs were reported [17–19].

Xu et al. reported 211 mW lasing at 1.92 μm in Tm:LuScO₃ ceramics with the slope efficiency of 8.2% [17]. Jing et al. reported 1 W lasing at 2.1 μm in a microchip Tm:(Lu_2/3Sc_1/3)₂O₃ ceramic with the slope efficiency of 24% [18]. Zhou reported 1.55 W lasing at 2.05 μm in Tm:LuYO₃ ceramics with the slope efficiency of 19.9% [19]. With the consideration of achievement of a lasing wavelength of Tm³⁺ longer than that in Lu₂O₃, the selection of (Lu,Sc)₂O₃ mixed crystals as a host is necessary rather than (Lu,Y)₂O₃. In the reported Tm: (Lu,Sc)₂O₃ mixed ceramics [17,18], the Sc/Lu compositions ratio are 1/1 and 1/2. A smaller Sc/Lu ratio has not been studied for evaluating its lasing performance.

In this paper, we report optical properties and 796 nm LD pumped 11 W CW lasing at 2.09 μm with 28.9% optical-to-optical conversion efficiency in Tm³⁺ doped Lu_1.6Sc_0.4O₃ mixed ceramics with the Sc/Lu ratio of 1/4.

2. Description of experiment

The ceramics of this work were fabricated in our lab using a solid-state reactive sintering method. Lu₂O₃, Sc₂O₃ and Tm₂O₃ (5 N) powders were used as the starting materials. ZrO₂ (99.5%) powder was used as a sintering aid. For fabrication of 1% Tm:Lu_1.6Sc_0.4O₃ and 1.5% Tm:Lu_1.6Sc_0.4O₃ ceramics, the raw materials were weighted with stoichiometric compositions of 0.99(0.8Lu₂O₃-0.2Sc₂O₃)-0.01Tm₂O₃ and 0.985(0.8Lu₂O₃-0.2Sc₂O₃)-0.015Tm₂O₃, respectively. In addition, the sintering aid was added with 3.0 wt%. The weighted powders were then milled in ethanol with zirconia balls for 48 h by a planetary ball-mill (RETSCH PM400, Germany). The ethanol solvent is removed by drying the milled slurry at 60 °C for 24 h and uniform granules are made by sieving through a 200 mesh screen. These sieved powders were calcined at 800 °C for 10 h and pressed into square plate shaped green bodies with a 20 × 25 mm² square stainless steel mold at 10 MPa and then cold isostatically pressed at 200 MPa. The green bodies were pre-sintered at 1800 °C for 15 h in vacuum (~10⁻³ Pa) for densification and then post-sintered by Hot Isostatic Pressing (HIP) for further densification at 1700 °C for 2 h in argon gas with gas pressure of 200 MPa. Finally, the sintered ceramics were annealed at 1500 °C for 10 h in air to eliminate the oxygen vacancies and remove internal stresses. Highly transparent ceramics can be achieved after optical polishing. After cutting, the square plate ceramics with the sizes of 2.6 × 15 × 18 mm³ are obtained for the study of optical properties, and the rectangular bars with the size of 2.6 × 2.6 × 15 mm³ for laser operation experiments.

The polycrystalline phase of the samples was analyzed by X-ray diffraction (XRD) (D8 Foucus, Bruker, Germany) using Cu Kα radiation. The morphology of the polished ceramic surface was characterized by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800). The optical transmission spectra were measured using a UV-VIS-NIR spectrometer (UV-3600plus, Shimadzu, Japan). The infrared emission spectra were recorded by a fiber optics optical meter (Ocean Optical, NIRQUEST256-2.5) under excitation of 800 nm LD. The samples were placed in a temperature controlled microscope stage (Linkam-THM S600, UK).

3. Results and discussion

The SEM morphologies of the thermal etching surface of the ceramics display pore-free patterns, as shown in Fig. 1(a). The grain boundaries are clean and no abnormal grains is observed. The average grain size is 1.54 μm. Figure 1(b) shows the XRD pattern of 1.5% Tm:Lu_1.6Sc_0.4O₃ ceramic together with the standard patterns of cubic Lu₂O₃ (JPCD cards. 12-0728) and Sc₂O₃(JPCD cards. 74-1210) for comparison. The measured pattern shows the identical structure to the standard patterns except the shift of diffraction angles, indicating pure cubic phase of the mixed sesquioxide ceramic. The XRD peaks of the ceramic are located between that of Lu₂O₃ and Sc₂O₃, reflecting the existence of Lu₂O₃ - Sc₂O₃ solid solution.

 

Fig. 1 Characterization of 1.5% Tm:Lu_1.6Sc_0.4O₃ ceramic. a: SEM morphology of the thermal etching surface and a inserted photo of a ceramic square plate (2.6 x 15 x 18 mm³) with 5 rectangular bars (2.6 x 2.6 x 15 mm³). b: XRD patterns. c: Transmission and emission spectra with an energy level diagram showing cross relaxation(CR) between two Er³⁺ ions and 2.1μm emissions due to a large Stark splitting of the ³H₆ ground states. d: The stimulated emission and absorption cross sections.

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Figure 1(c) illustrates the transmission spectrum for 1.5% Tm:Lu_1.6Sc_0.4O₃ ceramic plate with a thickness of 2.6 mm. A series characteristic absorption bands of Tm³⁺ appear in the range of 500 – 2300 nm, responsible for the transitions from the ground state ³H₆ to the excited states ³F_2,3, ³H₄, ³H₅ and ³F₄. High transmittances appear outside of the Tm³⁺ absorption bands. The transmittance at the lasing wavelength of 2.09 μm reaches 82.21% (circle), much close to the theoretical value (brackets) of 82.28% calculated by the modified Fresnel equation [20] T = 2n/(n² + 1), with n the refractive index of the ceramic. Using the refractive indices of 1.896 for Lu₂O₃ and 1.946 for Sc₂O₃ at 2 μm, we can determine the refractive index of the mixed ceramic to be 1.906 around 2μm if assuming a linear dependence of n on the composition in the 0.8Lu₂O₃-0.2Sc₂O₃ solid solution.

The ³F₄-³H₆ emission spectra are measured at room temperature (RT) and 77 K, respectively, upon ³H₄ excitation at 796 nm, as shown in Fig. 1(c). The 77 K spectrum covers a spectral range narrower than the RT one. At low temperature of 77 K the ³F₄ populations are considered to almost occupy the lowest Stark level of the ³F₄ manifold and the emission is contributed by the transition from the lowest Stark level of ³F₄ to the ³H₆ manifold. Hence, the spectral distribution range of the 77K emission reflects the Stark splitting of the ³H₆ manifold. In the 77K emission spectrum the emission peaks cover from 1760 to 2100 nm, indicating a 920 cm⁻¹ Stark splitting of the ³H₆ manifold with the transition peak between the lowest Stark levels of the ³F₄ and the ³H₆ states at 1760 nm. In the RT emission spectrum some additional emission peaks appear in the high energy side of the 77K emission, ranging from 1625 to 1760 nm. These additional emissions are attributed to the transitions from the upper Stark levels of ³F₄, which can be populated thermally as the temperature is raised from 77K to RT. As a result, the distribution range of these additional peaks indicates the range of Stark splitting of the ³F₄ states, which is 472 cm⁻¹ for the energy separation between 1625 and 1760 nm. Obviously, the large Stark splitting of the ³H₆ and ³F₄ states enable the ³F₄-³H₆ emission to contain a long wavelength emission band around 2.09 μm, originated from the transition from the lowest Stark level of ³F₄ to the highest Stark level of ³H₆, as illustrated in the energy level diagram of Tm³⁺ in Fig. 1(c). Moreover, the large Stark splitting of ³H₆ also results in low reabsorption losses around 2.09 μm, being favorite of laser operation. The cross-sections of the stimulated emission and the absorption are plotted together as shown in Fig. 1(d), wherein the cross-section of the emission is scaled to that of absorption for the transition (arrowed) between the lowest stark levels of the ³F₄ and the ³H₆ manifolds according to the results for Tm³⁺ doped (Lu,Sc)₂O₃ reported previously by Kränkel [21] and Jing [18]. One can see that the stimulated emission cross section at 2.09 μm is 1.1 × 10⁻²¹ cm² and the absorption cross section at 796 nm responsible for ³H₆-³H₄ transition is 3.1 × 10⁻²¹ cm².

The laser experimental setup is schematically shown in Fig. 2. The pump source was a fiber-coupled 796 nm AlGaAs diode laser (LIMO, Germany). A two-lens telescope with a magnification of 1:2 was used for image transfer of a pump beam from a 400-μm fiber output into the ceramic bars. The incoupling mirror (M1) had a high transmission at 796 nm and high reflection at 2090 nm. The output coupler (OC) was partially transmitting at 2090 nm and highly reflective at 796 nm, leading to a double pass of the pump radiation through the ceramic laser bars. The cavity length is 100 mm. The OC had a transmission of 2.3% at 2090 nm and a curvature radius of 120 mm. Tm: Lu_1.6Sc_0.4O₃ (1 or 1.5 at.% of Tm³⁺) ceramic bars with the sizes of 2.6 x 2.6 x 15 mm³ were used. Both facets in all the bars had an antireflective coating for the pump and lasing wavelengths at 796 nm and 2090 nm, respectively. The ceramic bars was wrapped with indium foil and fixed in a Cu radiator. The temperature of the radiator was stabilized by external water.

 

Fig. 2 Schematic of the ceramic laser setup with a cavity formed by an incoupling mirror (M1) and an output coupler (OC) pumped by a 796 nm laser diode.

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The gain cross-section, which can be calculated by [18]

where σ_g, σ_SE and σ_abs are the cross sections of gain, stimulated emission and absorption, respectively; β is the inversion ratio, defined as the population ratio of the ³F₄ to the ³H₆. Using the spectral data for the σ_SE and σ_abs shown in Fig. 1d, the gain cross-sections for various β are obtained from Eq. (1), as shown in Fig. 3a. One can see that the gain appears the maximum around 2090 nm for β less than 0.06. Figure 3(b) shows the output laser spectrum peaking at 2090 nm, the center of the gain curve.

 

Fig. 3 The gain cross sections (a) and output laser spectrum (b) of 1.5% Tm:Lu_1.6Sc_0.4O₃ ceramic. The pump power dependence of output power (c) and optical-to-optical conversion efficiencies (d) for 1% Tm and 1.5% Tm doped Lu_1.6Sc_0.4O₃ ceramics.

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Figure 3(c) depicts the pump power dependence of output power for 1% Tm³⁺ and 1.5% Tm³⁺ doped Lu_1.6Sc_0.4O ceramics. The corresponding optical-to-optical conversion efficiencies are shown in Fig. 3(d). An output power of 9.8 W is reached under 38 W pump for the 1% Tm³⁺ ceramic, indicating an optical-to-optical conversion efficiency of 25.8%. In the 1.5% Tm³⁺ ceramic, an 11 W output with 28.9% conversion efficiency under 38 W pump were achieved. One can also see that the slope efficiencies are 34.4% and 37.1% with respect to the pump power for 1% Tm³⁺ and 1.5% Tm³⁺, respectively. Since the ceramic absorbed approximately 85% of the pump light for 1% Tm³⁺, and 95% for 1.5% Tm³⁺, the corresponding slope efficiencies with respect to the absorbed pump power are about 40% and 39%, which are both larger than the theoretical limit (38%) for the conversion from one 796 nm photon to one 2090 nm photon. As a result, the cross relaxation between two Tm³⁺ ions for converting one 796 nm photon to two 2090 nm photons made a contribution to ³F₄ population. The observation of the cross relaxation effect indicates a high quality of the laser ceramics sintered in this work that the depletion of the population of the pump level ³H₄ through defects is suppressed. These output powers and efficiencies obtained in this work are the highest values reported so far for lasing around 2.1 μm in Tm³⁺ doped mixed sesquioxide ceramics pumped by a 796 nm diode. As the pump power is higher than 30 W, the deviation of the output power from the linear dependence is observed for both the ceramics. We attribute the phenomenon to thermal effect. The optimization of Tm³⁺ concentration and the ceramic length is necessary in the future study. The results of this work indicate that Tm³⁺ doped rare-earth mixed sesquioxide ceramics is a promising solid laser medium for 2.1 μm laser operation pumped by a 796 nm laser diode.

4. Summary

Highly transparent Lu_1.6Sc_0.4O₃ mixed sesquioxide ceramics doped with 1% Tm³⁺ or 1.5% Tm³⁺ has been fabricated by using a solid-state reactive sintering method, namely, pre-sintered in vacuum and then post-sintered by HIP. The polycrystalline ceramics consists of small grains with an average size of 1.54 μm. The transmittance at 2.09 μm reaches 82.21%, nearing the theoretical value of 82.28% at 2 μm. Large Stark splittings of 920 cm⁻¹ in the ground state ³H₆ manifold and 472 cm⁻¹ in the ³F₄ of Tm³⁺ are observed and they result in a long wavelength emission band around 2.09 μm due to transition from the lowest Stark level ³F₄ to the highest Stark level of ³H₆. The large Stark splitting of the ground states also minimize the reabsorption losses at 2.09 μm, making efficient and high power lasing at this wavelength possible. 11 W CW laser operation at 2.09 μm with an optical-to-optical conversion efficiency of 28.9% was achieved in 1.5% Tm:Lu_1.6Sc_0.4O₃ mixed ceramic pumped by a 796 nm LD, exhibiting its attractive application prospect.